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How do I evaluate guiding as guide scope focal length changes?

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#1 Linwood

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Posted 22 September 2020 - 11:02 PM

Same mount (CEM40), two different OTA's: 

 

1) C11 with OAG, coming in about 35# with the focal reducer

 

2) 400mm camera lens with 240mm guide scope (60mm aperture), probably 15# +/-.

 

While I am struggling a bit to fine tune, and I have issues occasionally, I am getting fair results with both.  But one aspect is not what I expected.

 

In the last two sessions, when things settled down: 

 

1) With the C11, operating at 2000mm, the total error was 0.51" (0.40 RA and 0.31" DEC) and peak errors of -2.12" RA and 1.11" DEC) 

 

2) With the short lens and 240mm guide scope, the guiding had 1.20" total error (0.84" RA and 0.85" Dec), but quite large peak errors (3.99" RA and 2.91 DEC). 

 

In other words, when I loaded the mount with a reasonable load, my guiding looks about half as good.  Of course, my guiding focal length also dropped by a factor of 8 or so. 

 

Neither of these are great, but the former is better than the seeing, and the latter is within one pixel on the imaging camera scale at 400mm.  So ... I think acceptable. 

 

But... does this kind of guiding error change based on focal length of the guide scope pass the sniff test?   Or do I need to work harder on the light load, shorter guide length? 

 

The guide camera in both cases is a ASI174MM-MINI by the way. 

 

Linwood



#2 michael8554

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Posted 23 September 2020 - 03:17 AM

OAG's are used to eliminate Differential Flexture caused by separate Guidescopes.

 

So how are your 400mm lens and 240mm Guidescope mounted ?



#3 SilverLitz

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Posted 23 September 2020 - 08:31 AM

It seems that my G11 also guides better with heavier loads than lighter. 

 

Last year, I got better guiding with my Esprit 100 and 60mm guidescope (~#22 total) compared to a much lighter 70-200 f/2.8L camera lens, but the camera lens setup was cross mounted on a Losmandy dovetail with the guidescope side-by-side.  I am trying a heavier/more complicated setup for the lenses mount directly to the Losmandy dovetail in a normal orientation and using large mounting rings to provide a platform for the guidescope to ride on top.  I hope that eliminating the lateral load will help.  

 

The best guiding I ever achieved with my Esprit is 0.7", but 0.9-1.2" is more typical and 1.5" on bad nights.  My worse nights are when it is hot/humid, and I expect worse seeing causing worse guiding.  On the bad nights PHD2's GA showed 0.7-0.8" high frequency movement.

 

My limited imaging of my new EdgeHD 925 w/ COAG, with total package ~34#, guided even better, 0.4-0.5".  This has me thinking about using an OAG (but not COAG) with my Esprit.



#4 Linwood

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Posted 23 September 2020 - 10:48 AM

OAG's are used to eliminate Differential Flexture caused by separate Guidescopes.

 

So how are your 400mm lens and 240mm Guidescope mounted ?

I would think that flexure would appear as longer period changes, e.g. in either very long exposures or undithered walking noise over a long period as the angle changes.  I guess it could appear as wind hits, but in comparing one night with a fair breeze and one with almost none, I see little difference (now that I have a decent tripod). 

 

But to answer the question, there's a very solid D style doveplate to which the lens foot is mounted, then a thick cross bar across the back to which the guide scope is mounted.  The shoe for the guide scope looks a bit strange because I mounted it off the edge so that the side provides a solid edge to prevent any twisting.

 

bars.jpg

 

I'm sure there could be some flex in all this metal hanging off the side, but I just don't see it occurring over the 3 second intervals randomly.

 

But of course, beautiful theories sometimes give way to inconvenient facts, so you may be right.  wink.gif

 

But the core of my question relates to the relative focal lengths.  Pretend for a moment I had the OAG AND the guidescope hooked up to identical loads, and pretend that there was just no flex anywhere.  Just for a moment pretend.

 

How should guiding accuracy change due to a change in focal length of the guiding from 2000mm to 240mm?   In equal circumstances with no other errors, how would guide error change? 



#5 StephenW

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Posted 23 September 2020 - 11:37 AM

Guiding at longer focal lengths gives the guider more detailed information on how the guide star is moving.

 

In theory, it should allow you to achieve "better" guiding, as you have more information.  How you use the information though is crucial.

 

In practice you may be at risk of just chasing the seeing or trying to "over guide" your mount (i.e. sending it too many guide corrections that it just can't mechanically handle).

 

So regardless of what your guiding focal length is, you need to make sure your guide corrections to your mount fit within your local seeing conditions and the mechanical constraints of your mount.


Edited by StephenW, 23 September 2020 - 11:40 AM.


#6 Linwood

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Posted 23 September 2020 - 11:50 AM

OK, all good advice. 

 

But what I think I'm asking for is math.  Does guide scope focal length (or arcsec/pixel) place some lower limit on expected guide accuracy that is quantifiable? 

 

I realize I can't just compare focal lengths simply, because there's also a lower limit on error based on the mount, seeing, hardware, etc.  So I am sure at 2000mm I'm limited by mechanics.

 

But is there an optical/calculation-in-PHD2 limit based on the optics as well? 



#7 StephenW

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Posted 23 September 2020 - 12:02 PM

>Does guide scope focal length (or arcsec/pixel) place some lower limit on expected guide accuracy that is quantifiable?

 

Yes.   The best I've seen PHD2 being able to guide / sustain is 0.20/pixel.  Maybe others have done better.  This is limited by PHD2's ability to accurately determine the centroid of the guide star within a partial pixel.

 

So, depending on your focal length and corresponding guiding imaging scale, the best RMS you are likely to achieve is around 1/5 of your guide pixel resolution.


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#8 Linwood

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Posted 23 September 2020 - 12:09 PM

So, depending on your focal length and corresponding guiding imaging scale, the best RMS you are likely to achieve is around 1/5 of your guide pixel resolution.

Ah, that is quite helpful.  Thank you.

 

At 2000mm that's 0.6"/pixel

 

At 240mm its 5"/pixel, arguing that the best I can get is 1".

 

So I'm probably never going to the less than 1" I get at 2000mm. 

 

And I'm not at all sure I am getting a clear image on the guide scope; that's on my list to investigate. The star images (in the big pane) have a lot of haze around them, like dew (there's no dew, I checked).  It's on the list.  Attempts to see a problem in daylight haven't yielded anything.

 

Like chasing guiding vs seeing, I don't want to chase it against an optical limit either.  Very helpful, thank you.



#9 SilverLitz

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Posted 23 September 2020 - 01:36 PM

Here is what my new lens mounting looks like.  The lens is a Samyang 135mm f/2.0.  My initial night with this guided better than last year's side by side arrangement.  I can put my HUGE EF300mm f/2.8L in this, but I run out of ring height, requiring my to eliminate the Arca-Swiss bracket and bolt directly to the Losmandy dovetail.

 

Samyang135_G11_04.JPG



#10 SilverLitz

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Posted 23 September 2020 - 01:39 PM

Here is what it looks like with HUGE EF300mm f/2.8L:

 

EF300mm_G11_04.JPG



#11 Linwood

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Posted 23 September 2020 - 01:47 PM

Here is what it looks like with HUGE EF300mm f/2.8L:

So just for the record, the 300/2.8 is just medium.  shocked.gif

 

I sure wish I could get my lens to accept a real AP camera.

 

I've thought about getting rings and mounting the guide scope above.  Right now it would be OK, but if real life ever returns I will be using the lens for sports shooting every few days, and I'm not wild about having to tear it all down and rebuild.  Right now it's just two screws. 

 

So how well does this guide?  How much better did it guide than side by side? 

 

I've really got to get out to a dark site and try a short lens; I have a nice 85/1.4 for example.



#12 Peregrinatum

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Posted 23 September 2020 - 01:56 PM

I'm always happy if total RMS < image scale, or less than 1.0

 

and...

 

stars are round



#13 SilverLitz

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Posted 23 September 2020 - 02:16 PM

So just for the record, the 300/2.8 is just medium.  shocked.gif

 

I sure wish I could get my lens to accept a real AP camera.

 

I've thought about getting rings and mounting the guide scope above.  Right now it would be OK, but if real life ever returns I will be using the lens for sports shooting every few days, and I'm not wild about having to tear it all down and rebuild.  Right now it's just two screws. 

 

So how well does this guide?  How much better did it guide than side by side? 

 

I've really got to get out to a dark site and try a short lens; I have a nice 85/1.4 for example.

It is as big a lens that I would ever use for AP, as I would then go for my Esprit 100 reduced to 413mm.  The Esprit's IQ  would be noticeably better than much more expensive camera lenses for AP.  

 

It is much easier doing AP with Canon lenses, as AstroMechanics has an adapter that converts the EF bayonet mount to M48 threads.  It also provides AF and aperture control through ASCOM.  The AstroMechanics adapter is much more mechanically secure than other lens adapters.

 

I bought the 1994 vintage EF300mm earlier this year to give me the shorter FL between 200-400mm.  I have yet to actually try it out, as my AstroMechanics adapter had a bad cable and I had to resort to manually focusing with a bahtinov mask, and the season changed to where my EdgeHD 925 and then Esprit 100 were better FLs.   I expect to try it out EF300 with my ASI183mm-Pro on M31 in a month.  It is going to be a tight fit, requiring the right camera angle, but the EF300 has a very nice rotation collar built in.

 

AstroMechanics replaced the entire setup a few months ago, and the new one worked great (testing in the house), but I found out the old one worked when used with the new cable.

 

A few days ago, this setup with the Samyang 135mm guided slightly better than my Esprit 100 had been guiding recently, so I would recommend the over/under instead of side by side.


Edited by SilverLitz, 23 September 2020 - 02:18 PM.


#14 Linwood

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Posted 23 September 2020 - 02:47 PM

It is as big a lens that I would ever use for AP, as I would then go for my Esprit 100 reduced to 413mm.  The Esprit's IQ  would be noticeably better than much more expensive camera lenses for AP.  

 

It is much easier doing AP with Canon lenses, as AstroMechanics has an adapter that converts the EF bayonet mount to M48 threads.  It also provides AF and aperture control through ASCOM.  The AstroMechanics adapter is much more mechanically secure than other lens adapters.

Sadly I learned after switching from Nikon to Sony Mirrorless the implications of mirrorless' short flange depth.  It's the flange depth that lets you use the Canon's like that, don't try it with their new mirrorless lenses.

 

I'll put my 400/2.8 up against your Esprit one day.  smile.gif

But then since I'm having to manually focus and use a non-AP camera, I have to get points as handicap.  lol.gif




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